a) Field of the Invention
This invention relates to a hetero-junction type bipolar transistor.
b) Description of the Related Art
Thinning a base to obtain a high-speed bipolar transistor leads to problems of an increase of a base resistance and of a decrease of a breakdown voltage. To prevent these problems, a high-concentration base can be used. This, however, causes other problems that a current gain is reduced and that a tunnel current is generated. One way to obtain a high-concentration base without these problems is to form a base-emitter hetero junction by the use of a narrow gap base or a wide gap emitter. It is known in the art that when an emitter is formed of a semiconductor having a greater forbidden band energy gap than an associated base, i.e., a wide-gap-emitter hetero junction, a current gain can be very high because when proper materials are selected the band gap of the emitter-base junction can be set not to a significant barrier against electrons but a substantial barrier against holes. Further, since a wide gap emitter can prevent a tunnel current which may be generated in the case of high-concentration junction, an emitter concentration can be reduced. Specifically, SiC is a typical material for a wide gap emitter when silicon is used for an associated base.
Another way to solve the above-mentioned problems is to use a narrow-band-gap-base hetero-junction type bipolar transistor in which a semiconductor having a band gap narrower than associated emitter is used as its base. In particular, the use of mixed crystals of SiGe as a base has been the subject of a great deal of work. It is well-known that a gradient in the concentration of Ge in the base by the use of mixed crystals of SiGe generates an electric field capable of accelerating minority carriers running through the base, to provide a high-speed device.
Further, it is also known that a bipolar transistor of still higher performance can be obtained when a base is an SiGe layer and an emitter is an SiC layer (See the hetero-junction type bipolar transistor 100 of FIG. 1, in which 106 is an SiGe layer and 108 is an SiC layer). In this modification, however, the base consists of a different semiconductor film from that of the emitter, so that mismatching in lattice takes place at their interface. As a result, a lattice defect and/or crystal deformation due to a stress occurs and then deteriorates the characteristics of the transistor, e.g., reduction in current gain.
To solve these problems in the hetero-junction type bipolar transistor 100, Japanese Patent Laid-Open No. 196432/1990 has proposed a hetero-junction type bipolar transistor 200 shown in FIG. 2 which is prepared by forming a silicon oxide film 230(or a silicon nitride film) on an Si base layer 206 and then forming an emitter layer, a polycrystalline semiconductor film 208 of a material other than Si over the film 230. In the hetero-junction type bipolar transistor 200, the silicon oxide film 230 (or a silicon nitride film) inserted between the Si base layer 206 and the polycrystalline semiconductor film 208 of the material other than Si prevents the different crystals from directly contacting each other, to reduce crystal defects around its interface due to lattice mismatching or a crystal deformation under a stress.
However, when SiGe is used as a base and SiC as an emitter, as shown in the hetero-junction type bipolar transistor 100 in FIG. 1, the difference in lattice constant is much more prominent than when Si (lattice constants; SiC=4.4 .ANG., SiGe=5.5 .ANG.) is used as a base, resulting that SiC may not epitaxially grow without dislocations at the interface.
When interposing a silicon oxide film or a silicon nitride film between a base and an emitter, as in the hetero-junction type bipolar transistor 200 illustrated in FIG. 2, the resistance of the emitter becomes too high and an epitaxial growth of an emitter on the base is restricted due to the presence of the insulating film between the emitter and the base, even though the thickness of the insulating film, 5-15 .ANG., is quite small.